Since polyphenylenesulfides having ##STR1## as basic units are excellent in properties, such as heat resistance, water resistance, chemical resistance, abrasion resistance, and electrical insulation, coatings containing polyphenylenesulfide as a base are used in the fields of rust prevention and corrosion protection of metals and electrical insulation work.
A coating containing polyphenylenesulfide as a base is generally applied in such a way that the coating is applied to the surface of an object in the form of a powder or slurry, and is heated to melt to form a coated film, or in such a way that the coating is sprayed in the form of a powder onto the surface of a preheated object to form a coated film. This is because polyphenylenesulfides do not dissolve in any organic solvent.
However, if a powder of polyphenylenesulfide is applied by using, for example, electrostatic spray coating, fluidized bed coating, or flame spray coating, which are usual methods of powder coating, a cissing formation phenomenon (a phenomenon wherein spots form discontinuous parts, or cratering, on the surface of coated film by the repellence of the coating composition) appears on the surface of the film when the powder is heated and melted. In many cases the cissing remains after curing and cooling, which makes it very difficult to form a uniform and smooth coated film.
The phenomenon not only injures the appearance of the film, but also causes pinholes, which greatly restricts the application of the coating. Therefore techniques for solving these problems are desired.
Despite the fact that the properties of polyphenylenesulfide coatings are excellent, the use of a coating containing polyphenylenesulfide as a base is extremely restricted for powder coating.
In order to prevent the cissing phenomenon, which occurs in the coating film when a polyphenylenesulfide composition is applied, an addition of metal oxide such as titanium dioxide to the composition was disclosed (e.g., Japanese Patent Publication No. 38689/1974 and Japanese Patent Application (OPI) No. 234067/1988). However, according to the inventor's tests the addition of the metal oxide for the prevention of cissing in the film is insufficient in such polyphenylenesulfide coatings.
As the cissing preventive agent, for example, acrylate polymers or copolymers, such as polyethyl acrylates, polybutyl acrylates, and poly-2-ethylhexyl acrylates, and fluorine-type surface-active agents, such as fluorinated alkyl esters, are widely used in conventional coatings. For example, Japanese Patent Application (OPI) No. 118468/1981 describes that a certain surface-active agent can be used for the prevention of cissing of epoxy resin powder coatings, and Japanese Patent Application (OPI) No. 193970/1984 describes that the addition of an acrylic oligomer, i.e., Modaflow (trade name, manufactured by Monsato Chemical Company, U.S.A.), to epoxy resin-type powder coatings is effective in the prevention of cissing.
In order to develop a polyphenylenesulfide composition for powder coating, the inventors first tried to apply several kinds of polyphenylenesulfide resin powders having different formulations onto metal plates by the usual powder coating technique.
As a result, a number of cissing formations occurred on the coating surface. Therefore, to prevent the cissing formation, known cissing preventive agents as described above were studied in regard to the kind and the amount to be added, which varies widely. However, it was found that the conventional general-purpose known cissing preventive agents for powder coating could not prevent cissing formation in the case of polyphenylenesulfide resin powders.
Cissing formation that occur in the coating surface during powder coating of a polyphenylenesulfide resin powder are, for example, due to the action of cohesion that is characteristic of polyphenylenesulfide resins, which takes place at the time of heating and melting after the application. The presence of such cissing adversely affects the long-term durability of the coating to a considerable extent.
The surface of a coating film obtained by powder coating a polyphenylenesulfide composition becomes orange-peel-like. Therefore, it is found that the smoothness of the coating film is insufficient when particularly fine smoothness is required.